Reddit reviews The Complete Grimm's Fairy Tales

Oooh, to be honest I'm not sure. I've not read anything similar to Fables that is 'younger'.

If he's interested in the legends and myths themselves, and he's a reader, maybe get him a collection of Grimm's Fairy Tales?

The Disney versions have been cleaned up a lot from the originals, but I'd guess that the Fables versions are at times 'worse' than the originals because of the additional themes in the narrative. If your nephew likes the more gory aspects... well Cindy's evil stepsisters get their eyes pecked out!

(I don't really know what the Grimms tales are like in aspects of nudity and so on, and the link above isn't a recommendation for that exact book, just as an idea. There's lots of versions of the Grimms stories nowadays, some more cleaned up than others.)